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Prior to this commit, there was only 1 global per-process
ThreadPoolExecutor used when thread_sensitive=True.

This results in tasks mixing sync/async effectively blocking once they
have contention for the sync thread.

This commit allows for the use of a task-specific thread executor. The
result is that async/sync mixed mode call chains will use exactly 2
threads in most cases:

* 1 asyncio thread (running the event loop)
* 1 sync thread (running any sync tasks)

This now guarantees that a request chain, for example, will re-use the
same sync thread.
11 contributors

Users who have contributed to this file

@andrewgodwin @tomchristie @untitaker @carltongibson @bjd183 @timb07 @jcass77 @blueyed @felixxm @basepi @a-feld
407 lines (360 sloc) 15.3 KB
import asyncio
import asyncio.coroutines
import functools
import os
import sys
import threading
import weakref
from concurrent.futures import Future, ThreadPoolExecutor
from .current_thread_executor import CurrentThreadExecutor
from .local import Local
try:
import contextvars # Python 3.7+ only.
except ImportError:
contextvars = None
def _restore_context(context):
# Check for changes in contextvars, and set them to the current
# context for downstream consumers
for cvar in context:
try:
if cvar.get() != context.get(cvar):
cvar.set(context.get(cvar))
except LookupError:
cvar.set(context.get(cvar))
class AsyncToSync:
"""
Utility class which turns an awaitable that only works on the thread with
the event loop into a synchronous callable that works in a subthread.
If the call stack contains an async loop, the code runs there.
Otherwise, the code runs in a new loop in a new thread.
Either way, this thread then pauses and waits to run any thread_sensitive
code called from further down the call stack using SyncToAsync, before
finally exiting once the async task returns.
"""
# Maps launched Tasks to the threads that launched them (for locals impl)
launch_map = {}
# Keeps track of which CurrentThreadExecutor to use. This uses an asgiref
# Local, not a threadlocal, so that tasks can work out what their parent used.
executors = Local()
def __init__(self, awaitable, force_new_loop=False):
self.awaitable = awaitable
try:
self.__self__ = self.awaitable.__self__
except AttributeError:
pass
if force_new_loop:
# They have asked that we always run in a new sub-loop.
self.main_event_loop = None
else:
try:
self.main_event_loop = asyncio.get_event_loop()
except RuntimeError:
# There's no event loop in this thread. Look for the threadlocal if
# we're inside SyncToAsync
main_event_loop_pid = getattr(
SyncToAsync.threadlocal, "main_event_loop_pid", None
)
# We make sure the parent loop is from the same process - if
# they've forked, this is not going to be valid any more (#194)
if main_event_loop_pid and main_event_loop_pid == os.getpid():
self.main_event_loop = getattr(
SyncToAsync.threadlocal, "main_event_loop", None
)
else:
self.main_event_loop = None
def __call__(self, *args, **kwargs):
# You can't call AsyncToSync from a thread with a running event loop
try:
event_loop = asyncio.get_event_loop()
except RuntimeError:
pass
else:
if event_loop.is_running():
raise RuntimeError(
"You cannot use AsyncToSync in the same thread as an async event loop - "
"just await the async function directly."
)
if contextvars is not None:
# Wrapping context in list so it can be reassigned from within
# `main_wrap`.
context = [contextvars.copy_context()]
else:
context = None
# Make a future for the return information
call_result = Future()
# Get the source thread
source_thread = threading.current_thread()
# Make a CurrentThreadExecutor we'll use to idle in this thread - we
# need one for every sync frame, even if there's one above us in the
# same thread.
if hasattr(self.executors, "current"):
old_current_executor = self.executors.current
else:
old_current_executor = None
current_executor = CurrentThreadExecutor()
self.executors.current = current_executor
# Use call_soon_threadsafe to schedule a synchronous callback on the
# main event loop's thread if it's there, otherwise make a new loop
# in this thread.
try:
awaitable = self.main_wrap(
args, kwargs, call_result, source_thread, sys.exc_info(), context
)
if not (self.main_event_loop and self.main_event_loop.is_running()):
# Make our own event loop - in a new thread - and run inside that.
loop = asyncio.new_event_loop()
loop_executor = ThreadPoolExecutor(max_workers=1)
loop_future = loop_executor.submit(
self._run_event_loop, loop, awaitable
)
if current_executor:
# Run the CurrentThreadExecutor until the future is done
current_executor.run_until_future(loop_future)
# Wait for future and/or allow for exception propagation
loop_future.result()
else:
# Call it inside the existing loop
self.main_event_loop.call_soon_threadsafe(
self.main_event_loop.create_task, awaitable
)
if current_executor:
# Run the CurrentThreadExecutor until the future is done
current_executor.run_until_future(call_result)
finally:
# Clean up any executor we were running
if hasattr(self.executors, "current"):
del self.executors.current
if old_current_executor:
self.executors.current = old_current_executor
if contextvars is not None:
_restore_context(context[0])
# Wait for results from the future.
return call_result.result()
def _run_event_loop(self, loop, coro):
"""
Runs the given event loop (designed to be called in a thread).
"""
asyncio.set_event_loop(loop)
try:
loop.run_until_complete(coro)
finally:
try:
# mimic asyncio.run() behavior
# cancel unexhausted async generators
if sys.version_info >= (3, 7, 0):
tasks = asyncio.all_tasks(loop)
else:
tasks = asyncio.Task.all_tasks(loop)
for task in tasks:
task.cancel()
loop.run_until_complete(asyncio.gather(*tasks, return_exceptions=True))
for task in tasks:
if task.cancelled():
continue
if task.exception() is not None:
loop.call_exception_handler(
{
"message": "unhandled exception during loop shutdown",
"exception": task.exception(),
"task": task,
}
)
if hasattr(loop, "shutdown_asyncgens"):
loop.run_until_complete(loop.shutdown_asyncgens())
finally:
loop.close()
asyncio.set_event_loop(self.main_event_loop)
def __get__(self, parent, objtype):
"""
Include self for methods
"""
func = functools.partial(self.__call__, parent)
return functools.update_wrapper(func, self.awaitable)
async def main_wrap(
self, args, kwargs, call_result, source_thread, exc_info, context
):
"""
Wraps the awaitable with something that puts the result into the
result/exception future.
"""
if context is not None:
_restore_context(context[0])
current_task = SyncToAsync.get_current_task()
self.launch_map[current_task] = source_thread
try:
# If we have an exception, run the function inside the except block
# after raising it so exc_info is correctly populated.
if exc_info[1]:
try:
raise exc_info[1]
except Exception:
result = await self.awaitable(*args, **kwargs)
else:
result = await self.awaitable(*args, **kwargs)
except Exception as e:
call_result.set_exception(e)
else:
call_result.set_result(result)
finally:
del self.launch_map[current_task]
if context is not None:
context[0] = contextvars.copy_context()
class SyncToAsync:
"""
Utility class which turns a synchronous callable into an awaitable that
runs in a threadpool. It also sets a threadlocal inside the thread so
calls to AsyncToSync can escape it.
If thread_sensitive is passed, the code will run in the same thread as any
outer code. This is needed for underlying Python code that is not
threadsafe (for example, code which handles SQLite database connections).
If current_context_func is passed, the code will run 1 thread per context.
As an example, this may be used to create a per-request synchronous thread
by specifying the request object as the context. Thread scheduling will
occur by request in this scenario - each request will execute synchronous
work within the same thread.
If the outermost program is async (i.e. SyncToAsync is outermost), then
this will be a dedicated single sub-thread that all sync code runs in,
one after the other. If the outermost program is sync (i.e. AsyncToSync is
outermost), this will just be the main thread. This is achieved by idling
with a CurrentThreadExecutor while AsyncToSync is blocking its sync parent,
rather than just blocking.
"""
# If they've set ASGI_THREADS, update the default asyncio executor for now
if "ASGI_THREADS" in os.environ:
loop = asyncio.get_event_loop()
loop.set_default_executor(
ThreadPoolExecutor(max_workers=int(os.environ["ASGI_THREADS"]))
)
# Maps launched threads to the coroutines that spawned them
launch_map = {}
# Storage for main event loop references
threadlocal = threading.local()
# Single-thread executor for thread-sensitive code
single_thread_executor = ThreadPoolExecutor(max_workers=1)
# Maintaining a weak reference to the context ensures that thread pools are
# erased once the context goes out of scope. This terminates the thread pool.
context_to_thread_executor = weakref.WeakKeyDictionary()
def __init__(self, func, thread_sensitive=True, current_context_func=None):
self.func = func
functools.update_wrapper(self, func)
self._thread_sensitive = thread_sensitive
self._is_coroutine = asyncio.coroutines._is_coroutine
self._current_context_func = current_context_func
try:
self.__self__ = func.__self__
except AttributeError:
pass
async def __call__(self, *args, **kwargs):
loop = asyncio.get_event_loop()
# Work out what thread to run the code in
if self._thread_sensitive:
if hasattr(AsyncToSync.executors, "current"):
# If we have a parent sync thread above somewhere, use that
executor = AsyncToSync.executors.current
elif self._current_context_func:
# If we have a way of retrieving the current context, attempt
# to use a per-context thread pool executor
current_context = self._current_context_func()
if current_context in self.context_to_thread_executor:
# Re-use thread executor in current context
executor = self.context_to_thread_executor[current_context]
else:
# Create new thread executor in current context
executor = ThreadPoolExecutor(max_workers=1)
self.context_to_thread_executor[current_context] = executor
else:
# Otherwise, we run it in a fixed single thread
executor = self.single_thread_executor
else:
executor = None # Use default
if contextvars is not None:
context = contextvars.copy_context()
child = functools.partial(self.func, *args, **kwargs)
func = context.run
args = (child,)
kwargs = {}
else:
func = self.func
# Run the code in the right thread
future = loop.run_in_executor(
executor,
functools.partial(
self.thread_handler,
loop,
self.get_current_task(),
sys.exc_info(),
func,
*args,
**kwargs
),
)
ret = await asyncio.wait_for(future, timeout=None)
if contextvars is not None:
_restore_context(context)
return ret
def __get__(self, parent, objtype):
"""
Include self for methods
"""
return functools.partial(self.__call__, parent)
def thread_handler(self, loop, source_task, exc_info, func, *args, **kwargs):
"""
Wraps the sync application with exception handling.
"""
# Set the threadlocal for AsyncToSync
self.threadlocal.main_event_loop = loop
self.threadlocal.main_event_loop_pid = os.getpid()
# Set the task mapping (used for the locals module)
current_thread = threading.current_thread()
if AsyncToSync.launch_map.get(source_task) == current_thread:
# Our parent task was launched from this same thread, so don't make
# a launch map entry - let it shortcut over us! (and stop infinite loops)
parent_set = False
else:
self.launch_map[current_thread] = source_task
parent_set = True
# Run the function
try:
# If we have an exception, run the function inside the except block
# after raising it so exc_info is correctly populated.
if exc_info[1]:
try:
raise exc_info[1]
except Exception:
return func(*args, **kwargs)
else:
return func(*args, **kwargs)
finally:
# Only delete the launch_map parent if we set it, otherwise it is
# from someone else.
if parent_set:
del self.launch_map[current_thread]
@staticmethod
def get_current_task():
"""
Cross-version implementation of asyncio.current_task()
Returns None if there is no task.
"""
try:
if hasattr(asyncio, "current_task"):
# Python 3.7 and up
return asyncio.current_task()
else:
# Python 3.6
return asyncio.Task.current_task()
except RuntimeError:
return None
# Lowercase aliases (and decorator friendliness)
async_to_sync = AsyncToSync
def sync_to_async(func=None, thread_sensitive=True, current_context_func=None):
if func is None:
return lambda f: SyncToAsync(
f,
thread_sensitive=thread_sensitive,
current_context_func=current_context_func,
)
return SyncToAsync(
func,
thread_sensitive=thread_sensitive,
current_context_func=current_context_func,
)